In general, efforts in the art to provide digital logic circuits for integrated circuitry have developed into a two stage type of circuit in which in a first stage, the logic portion of the circuit has the output thereof developed across a non-linear load which provides a very clear signal level in one signal direction and that signal level then actuates a second, driving stage which performs such functions as assisting in establishing the upper, lower and signal shape limits of the digital output signal and at the same time that driving stage provides sufficient power to achieve the speed in driving the types of loads associated with subsequent logic stages.
One illustration of the two stage type of digital logic circuit is in U.S. Pat. No. 4,028,556. In this circuit, in the first or logic stage, enhancement mode type field effect transistors with one electrode connected to ground are employed for the signal input devices for the digital logic variable signals and a depletion mode type field effect transistor with the gate thereof connected to the source is employed as the non-linear load. In the second or driving stage, two depletion mode type field effect transistors are connected with their sources and drains in series between two separate above and below ground voltages. The signal from the logic stage is introduced at the gate of one of the depletion mode devices. The second depletion mode type field effect transistor is connected with the gate thereof connected to the below ground voltage. The output is taken from between the devices. In such a construction, a diode is needed to provide a signal level shift.
Another illustration of the two stage type of digital logic circuit is shown in U.S. Pat. No. 4,405,870. This circuit in the first stage employs Schottky diodes as digital logic variable input devices and uses a field effect transistor with the source and the gate connected as the non-linear load. The non-linear load transistor is connected to a negative voltage. A diode that provides a level shifting function is employed between the logic input devices and the load.
In the driving stage, two series source to drain connected field effect transistors are connected between ground and a positive voltage with the output taken between them, the gate of the one at the positive voltage is connected to the output and the logic stage output is connected to the gate of the field effect transistor connected to the ground voltage.
In each instance in the art, voltages on both sides of ground and level shifting is needed and the additional devices for level shifting make it more difficult to retain yield and meet the ever increasing device density requirements.